1. Field of the Invention
The present invention relates to a process for metal finishing of aerospace alloys for structural adhesive bonding and corrosion protection. More particularly, the process comprises applying an aqueous solution containing an epoxy-functional organosilane and a cross-linking silane to a clean or etched surface of an aerospace alloy. The surface is etched without grit blasting.
Almost all metals are subject to corrosion, particularly atmospheric corrosion. This corrosion affects the quality of the metals, and hence the quality of the final products prepared from the metals. Also, removal of corrosion from the metal surface is both time consuming and economically unfavorable. If it is desired to coat the metal surface with a paint, an adhesive or a rubber, then corrosion of the base metal can cause poor adhesion of paint, adhesive or rubber to the metal. With poor adhesion of a coating, the final product is subject to further corrosion.
2. Description of the Related Art
In the prior art, processes for improving the corrosion resistance of metals included passivating the metal surface by means of a chromate treatment. However, chromium is proven to be highly toxic, carcinogenic and harmful to the environment. It has also been disclosed to employ a phosphate conversion coating in conjunction with a chromate rinse to provide corrosion resistance to a metal surface such as aluminum and magnesium. However, even minor use of chromium should be avoided. In general, phosphate conversion coating is not effective without the chromate rinse.
In recent years, use of chromium has been working toward elimination in favor of a treatment for metal surfaces which comprises coating the metal surface with an inorganic silicate followed by treating the silicate coating with an organofunctional silane (U.S. Pat. No. 5,108,793). Another process involves the rinsing of an aluminum sheet with a solution containing an organofunctional silane and a cross-linking agent to provide temporary corrosion protection. A dense siloxane film can be formed on the metal (U.S. Pat. No. 5,292,549). However, the organofunctional silane does not bond well to the metal surface, and the dense siloxane film can be easily rinsed off.
Many other techniques for corrosion protection of metal surfaces including aerospace alloys, have been disclosed. Some of these techniques are as follows:
U.S. Pat. No. 6,579,472 to Chung et al. discloses an anti-corrosive sol-gel that is a combination of an organometallic salt, an organosilane, and a borate, phosphate, or zinc functional component. The functional components of the sol-gel are combined with an organic acid prior to its application to metal.
U.S. Pat. No. 6,403,164 to Jonschker et al. discloses a process for protecting a metallic substrate against corrosion by coating with a composition having a polysiloxane or heteropolysiloxane and having a component derived from at least one metal of the metallic substrate.
U.S. Pat. No. 5,660,884 to Crook et al. discloses a two-step process for treating a titanium or titanium alloy surface with an alkaline aqueous silicate solution. A solution of an alkali metal metasilicate is preferred because of its buffering capacity. The '884 patent discloses a silicate ratio of about 0.004 molar to about 0.04 molar.
U.S. Pat. No. 5,292,549 to van Ooij et al. discloses a metallic coated steel sheet having a thin siloxane film for providing temporary corrosion protection. The siloxane film is formed by rinsing the metallic coated steel sheet with an organic silane and a cross-linking agent. In the '549 patent, the silane coating is formed from a solution containing 0.5–2.0 vol. % of silane and 0.1–1.0 vol. % of cross-linking agent.
None of the above-referenced patents, taken either individually or in combination, anticipate the present invention as claimed.